1. Field of the Invention
The present invention relates to a wire electric discharge machine, and more particularly, to a wire electric discharge machine which detects a tensile force of a wire electrode and performs feedback control of the wire electrode.
2. Description of the Related Art
In a wire electric discharge machine, an appropriate tensile force is applied to a wire electrode and electric discharge is carried out. By applying the tensile force to the wire electrode, vibration of the wire electrode is suppressed and precise machining is obtained. However, if the tensile force applied to the wire electrode is excessively strong, there is fear that the wire electrode is cut and the machining is interrupted. If the tensile force applied to the wire electrode is excessively weak on the other hand, vibration of the wire electrode cannot be suppressed, and the machining precision is deteriorated. Hence, it is necessary to apply an appropriate tensile force to the wire electrode during the machining.
As methods of applying a tensile force to a wire electrode, there is a method in which a wire electrode is wound around a brake roller connected to an electromagnetic brake, or a wire electrode is pressed against a brake roller by a pinch roller, and voltage applied to the electromagnetic brake is controlled, thereby controlling the tensile force to be applied to the wire electrode, and there is also another method in which rollers for guiding a running state of a wire electrode are placed upstream and downstream of a wire electrode which runs such that the rollers sandwich an electric discharge region, and speed or a torque of a motor which drives the rollers are controlled, thereby controlling a tensile force of the wire electrode.
Optimal tensile force applied to a wire electrode differs depending upon a wire diameter and a kind of the wire electrode and a kind of machining, and a range of magnitude of the tensile force to be set is wide. However, it is difficult to precisely set a minute or fine difference of the tensile force. Hence, there is used a method in which a tensile force of a wire electrode is measured using a tensile force detector, and the tensile force of the wire electrode is feedback controlled based on the measured tensile force.
For example, JP 4230157 B1 describes an invention that a brake roller placed upstream of a wire electrode which runs and a lower roller placed downstream of the wire electrode sandwich a machining region, a wire electrode is wound between the brake roller and the lower roller, a tensile force detector for detecting a tensile force of the wire electrode between the brake roller and the lower roller is provided, a speed command is output to a recovery motor which drives a recovery roller placed below the lower roller, thereby controlling the speed of the recovery motor and driving the same, a current command is produced for a brake motor which drives the brake roller based on a tensile force command signal, based on a tensile force detection signal detected by the tensile force detector, based on a speed command to the brake motor, and based on a speed detection signal from a speed detector of the brake motor, and the current command is output to the brake motor to control (torque control) the same, thereby controlling the tensile force of the wire electrode.
Further, JP 3416514 B1 describes an invention that a take-back motor which takes back a wire electrode is driven at predetermined speed, a speed command for a brake motor is produced based on a tensile force setting signal, a tensile force actually measured value detected by a tensile force detector, and a speed setting signal, and the speed of the brake motor is controlled by the produced speed command, thereby controlling the tensile force of the wire electrode.
Furthermore, JP 2010-179377 A describes an invention that a tensile force detecting device for detecting a tensile force of a wire electrode which runs in a machining region is provided, and speed or torque of a brake motor which drives a brake roller placed upstream of a running state of the wire electrode, or speed or torque of a sending-out motor for driving a roller which sends out the wire electrode placed downstream of the running state of the wire electrode are controlled such that the tensile force detected by the tensile force detecting device becomes equal to a set tensile force, thereby controlling the tensile force of the wire electrode.
A method of detecting a tensile force of a wire electrode by a tensile force detector and feedback controlling the tensile force of the wire electrode using the detected tensile force is generally employed. However, although the tensile force detector can measure variation in the tensile force by fine resolution, a range where the tensile force can precisely be detected is small. Further, a detected value is varied depending upon a kind of the tensile force detector and depending upon temperature variation, and the tensile force cannot be measured precisely and as a result, there is a problem that a tensile force cannot precisely be controlled. There is a method of calibrating a tensile force detector using a thermometer, but since individual variability exists in the tensile force detectors, it is necessary to perform calibration for every tensile force detector, and there is a problem that the number of operation steps is increased.